30 May 2012

What’s that rock made of? Good question…

Posted by Ryan Anderson

What's this rock made of?

You would think it would be pretty easy to figure out what a rock is made of, but it is actually a very complicated question. For ChemCam, we need a list of geologic samples (“standards”) with a range of known compositions so that we can accurately measure the composition of unknown rocks on Mars. Our measurements are only as good as our standards, so I have been spending an inordinate amount of time lately working with another post-doc on the team, making sure that the database of compositions is accurate. (This was one of those cases where you say “Hey, someone really needs to do that thing” and then you realize that “someone” might as well be you.)

One of the first complications in compiling a table of standards is that these standards are real geological materials. Yes, they are selected and prepared so that they are as uniform as possible, but slight variations in the samples and in the laboratories doing the analysis lead to slightly different compositions, depending who you ask. The best references give the combined results from many different institutions but some standards are only available from one source and you just have to trust it, or do your own analysis.

Another complication is that the most common method of measuring rock composition, x-ray fluorescence (XRF), is not sensitive to elements much lighter than sodium. That’s fine for a basalt that is full of iron and silicon and magnesium, but we are going to Gale crater because it has evidence of water and alteration. It is notoriously difficult to measure water in geologic samples. XRF can’t see the hydrogen or the oxygen, so that doesn’t do any good. And even if you use methods that can measure light elements, hydrated minerals have the annoying habit of interacting with the water in the atmosphere. So a sample analyzed in the high desert at Los Alamos might have much less water in it than the same sample analyzed in Houston. That means that in a lot of cases, there just isn’t any information about how much water is in a sample.

Oxygen is the most abundant element in the crust of rocky planets, so the rocks are mostly made of oxides.

And finally, one of my biggest pet peeves about geochemistry is that the compositions of rocks are always expressed as oxides. So instead of reporting that a certain rock has 10% Si and 5% Mg, all of the references report percentages of SiO2 and MgO. That’s all well and good in most cases, since most rocks are made of oxides. But then you come across samples that aren’t all oxidized, and when you convert the composition to oxides the percentages don’t add up. For example, one of the samples in our database is an oil shale. The fact that the rock contains oil means that the carbon isn’t all bonded to oxygen, so when you express the carbon as CO2 and then add up the oxides, you get a ridiculous total. The rock in question added up to something like 160%

There are a few elements that have this problem. Carbon can exist in rocks as C or CO2, sulfur is usually in the form of SO3, but sometimes as S, and then there’s iron. I hate iron. Which is a shame, because it is a really common element in rocks, and is responsible for the lovely black and red coloring of the martian surface (and in most other rocks for that matter – iron is a strong pigment). I hate iron because it commonly exists as FeO AND Fe2O3 in the same rock. That would be fine if both of those were reported in the references, but usually they just report iron as if it exists only as Fe2O3. But then sometimes a source will list all of the iron as FeO. And then to add another layer of confusion, some types of rocks such as fool’s gold or iron meteorites have reduced iron: Fe.

This pyrite is 63.5% Fe and 36.5% S. As oxides that would be 90.8% Fe2O3 and 91.1% SO3.

But I am happy to say that, after many tedious days of copying and double-checking all of the compositions, the table is coming together. We’re down to a handful of troublesome samples that need to be dealt with individually. (For example I found out today that one of our samples called WMG is completely different from the geostandard called WMG-1. No, not confusing at all!) The current table will provide the foundation for our analyses on Mars, and as time goes on, we’ll add more standards to the table which will allow us to continue to improve our accuracy.